The present invention relates to a spring-loaded torque limiter used for office machines such as printers and copiers, especially compact ones.
FIG. 8 shows a conventional spring-loaded torque limiter used for office machines (as disclosed in FIGS. 1 and 2 of JP patent publication 8-270673). It includes an annular outer member 31 having a boss portion 32 at one end thereof. An inner ring 33 is inserted in the annular outer member 31 and supported by the boss portion 32 so as to be rotatable relative to the annular outer member 31. A coil spring 34 is interference-fitted on the radially outer surface of the inner ring 33. A lid member 39 is fitted on the radially inner surface of the annular outer member 31 at the other end thereof to close a gap between the radially inner surface of the annular outer member 31 and the radially outer surface of the inner ring 33. The coil spring 34 comprises a large-diameter portion 34a and a small-diameter portion 34b interference-fitted on the radially outer surface of the inner ring 33. From the end of the small-diameter portion 34b, a hook 35 extends axially and engages the boss portion 32 of the annular outer member 31. From the end of the large-diameter portion 34a, a hook 36 extends axially and engages the lid member 39.
When the inner ring 33 is turned relative to the outer member 31 in the direction opposite to the direction in which the coil spring is wound as viewed from the right-hand side of FIG. 8, the small-diameter portion 34b is unwound slightly so that the turning resistance between the small-diameter portion 34b and the inner ring 33 decreases. Thus, torque greater than the turning resistance between the small-diameter portion 34b and the inner ring 33 is not transmitted between the inner ring 33 and the outer member 31. When the inner ring 33 is turned in the direction in which the coil spring is wound, the small-diameter portion 34b will be wound further tightly, so that the turning resistance between the small-diameter portion 34b and the inner ring 33 increases to such a level that the small-diameter portion 34b and the inner ring 33 are practically locked together. Thus, when the inner ring 33 is turned in the direction in which the coil spring is wound, the torque limiter loses its torque-limiting function. In order to adjust the turning resistance between the small-diameter portion 34b and the inner ring 33, which determines the maximum torque that can be transmitted between the inner ring 33 and the outer member 31, the lid member 39 is turned with the annular outer member 31 fixed to adjust the interference with which the small-diameter portion 34b is fitted on the inner ring 33.
The torque limiter 30 is mounted to a paper feed roller 37 of e.g. an office machine by coupling the end of the annular outer member 31 where there is the boss portion 32 to a roller body 38 of the roller 37. While the torque applied to the roller 37 is smaller than the set torque, the inner ring 33 and the roller 37 rotate together. If the torque applied to the roller 37 exceeds the set torque, the inner ring 33 slips, thus limiting transmission of torque.
Torque limiters including a coil spring having a uniform diameter are also known (see FIG. 1 of JP patent publication 10-110739 and FIGS. 1 and 2 of JP patent publication 11-257368). The coil spring disclosed in publication 10-110739 has at either end thereof a radially outwardly extending hook that engages the annular outer member. The coil spring disclosed in publication 11-257368 has an axially extending hook at one end thereof.
Because the coil spring 34 of the torque limiter 30 shown in FIG. 8, i.e. the torque limiter disclosed in JP patent publication 8-270673, includes the large-diameter portion 34a besides the small-diameter portion 34b to be interference-fitted on the inner ring 33, the torque limiter of FIG. 8 tends to be large in size. Since it is necessary to adjust the maximum torque that can be transmitted by turning the lid member 39, the cost for mass-production tends to be high. Because the coil spring of JP patent publication 10-110739 has the radially outwardly extending hooks, the annular outer member of the torque limiter disclosed in this publication has to have a correspondingly large diameter. Because the coil spring of JP patent publication 11-257368 has the axially outwardly extending hook, the annular outer member of the torque limiter disclosed in this publication tends to be large in axial length. Thus, the torque limiters disclosed in these publications tend to be large in size. Moreover, since the torque limiter disclosed in either publication has to be assembled so that the end faces engage the respective engaging portion formed on the annular outer member, productivity is low, and thus the production cost tends to be high.
An object of the present invention is to provide a torque limiter which is small in size and can be manufactured at a low cost.